How to Choose a Pyranometer for Solar PV Monitoring

Choose a pyranometer by matching the measurement objective, required performance, mounting plane and monitoring-system interface. For solar PV projects, the first decision is whether you need global horizontal irradiance, irradiance in the plane of the modules, or additional measurements for a bifacial system.

RIKA SENSOR offers two different measurement approaches: the rk200-03 thermopile pyranometer series, available in Class A, B and C versions, and the RK200-04 silicon photodiode solar radiation sensor. Understanding their differences helps you select an instrument that fits the project's data requirements and operating conditions.

A pyranometer measures solar irradiance received by its sensing plane, expressed in watts per square metre, or W/m². Its mounting position determines what the measurement represents.

Global horizontal irradiance: GHI

GHI is the solar irradiance received on a horizontal surface. It includes the direct solar contribution projected onto that surface and diffuse radiation from the sky.

Use a horizontal, level installation when the project requires GHI for solar-resource monitoring, weather observations or comparison with horizontal irradiance datasets.

Plane-of-array irradiance: POA

POA irradiance is received in the plane of the PV modules. For a fixed-tilt array, the sensor should match the modules' tilt and azimuth. For a tracking array, its orientation should follow the intended measurement plane.

POA measurements help relate the sunlight reaching the modules to electrical output. A horizontal sensor and a tilted sensor measure different quantities, so their readings should not be treated as interchangeable.

Bifacial and reflected-radiation measurements

Bifacial projects may need rear-side irradiance or ground-reflected radiation measurements in addition to front-side POA measurements.

Define these measurements separately. A downward-facing sensor used for ground-reflected radiation serves a different purpose from a sensor positioned to represent irradiance reaching the rear of the modules.

The number and distribution of sensors should reflect the monitoring specification, array layout, terrain and variation across the site.

The detector technology affects spectral response and how readings behave under different sunlight conditions.

RK200-03: thermopile measurement

The RK200-03 uses a thermopile detector. Solar radiation heats an absorbing surface, producing a temperature difference that generates an electrical signal.

RIKA's V5.0 specifications list:

• Class A and B: 280-3000 nm spectral range.
• Class C: 300-3200 nm spectral range.
• Class-dependent response time, non-linearity, temperature effect and annual stability.

This broad spectral coverage makes the RK200-03 series a suitable product family to evaluate when a project calls for broadband solar irradiance measurement and a specified ISO 9060 class.

RK200-04: silicon photodiode measurement

The RK200-04 uses a silicon photodiode detector with a specified spectral range of 300-1100 nm. Its V5.0 specifications include:

• Measurement range: 0-1500 W/m².
• Resolution: 1 W/m².
• Power consumption: 0.2 W.
• IP67 ingress protection.
• Multiple analog and digital output options.

Its narrower spectral response means that interpretation depends more on the relationship between the sensor's calibration conditions and the sunlight being measured. Evaluate this characteristic when comparing readings across changing atmospheric conditions or against a broadband thermopile instrument.

The RK200-04 datasheet does not state an ISO 9060 class. Projects requiring a particular class should select an instrument with that classification documented for the ordered configuration.

ISO 9060 instrument classes and IEC 61724-1 monitoring-system classes describe different things.

ISO 9060:2018 classifies solar radiation instruments according to performance characteristics. IEC 61724-1:2021 addresses photovoltaic performance monitoring systems, including equipment and measurement requirements.

An instrument's class is therefore one part of the monitoring-system selection. Installation, calibration, environmental conditions and operating procedures also affect the resulting data.

Comparing RK200-03 Class A, B and C

The following values come from RIKA SENSOR's RK200-03 V5.0 specifications.

These figures describe the RK200-03 versions; they are not a table of universal ISO class limits. Non-linearity is also a separate specification from total measurement uncertainty.

Within this product family, Class A is the starting point for the most demanding performance requirements. Class B and Class C can be evaluated where their documented performance satisfies the monitoring plan.

For a project specifying a descriptor such as spectrally flat Class A, request documentation covering that complete requirement. Include the calibration uncertainty and any required dew or frost mitigation in the technical review.

Choose the interface before placing the order. The sensor must work with the available power supply, data logger, PLC or monitoring controller.

These are configuration options; the table does not imply that every output is available simultaneously.

The RK200-03 specifications identify Modbus RTU for its RS485 interface. For the RK200-04, request the communication documentation for the selected version.

Check the transmitter arrangement

For RK200-03 Class A and B, the datasheet lists an external transmission module for 4-20 mA and a built-in module for RS485.

For Class C, RS485 and SDI-12 are listed as built-in, while 4-20 mA and 0-5 V use external transmission modules. Include any required module, enclosure space and wiring in the installation plan.

Confirm scaling during commissioning

For an unamplified RK200-03, calculate irradiance using the individual instrument's sensitivity coefficient:

Irradiance in W/m² = output in µV ÷ sensitivity in µV/(W/m²)

For analog transmitters, configure the logger using the irradiance span confirmed for the ordered sensor and transmitter combination.

Also check the complete acquisition chain. Detector response, communication updates, logger sampling and recorded averages are separate settings that influence the final dataset.

A correctly specified instrument needs a representative location and a practical maintenance plan.

Match the intended measurement plane

For GHI, use the leveling arrangement to establish a horizontal sensing plane. For POA, align the sensor with the relevant module plane.

Check for shadows from mounting structures, nearby instruments, cables and vegetation throughout the day. Record the installation position and orientation so later changes can be identified.

The RK200-03 series offers optional horizontal or adjustable-angle brackets, together with leveling features for installation.

Assess dew, frost and contamination

Both product datasheets list IP67 protection. This describes enclosure protection; it does not prevent dew, frost, dust or bird contamination from affecting the optical surface.

Sites exposed to condensation or freezing conditions should assess the need for heating, ventilation or other suitable mitigation. Confirm accessory compatibility and the available power budget during selection.

Provide access for servicing

For the RK200-03, RIKA's maintenance instructions call for regular glass-dome checks and dry desiccant. They recommend replacing desiccant when its indicator changes colour, with a six-month replacement recommendation.

Set inspection and cleaning schedules according to local conditions and the monitoring specification. Position instruments so technicians can service them without disturbing their alignment.

When comparing quotations, include calibration, cleaning access, replacement parts and service arrangements in the lifetime cost.

Before ordering a pyranometer for solar PV monitoring, confirm:

1. Monitoring objective: Resource assessment, operational trends, performance analysis or contractual reporting.
2. Measurement plane and locations: GHI, front-side POA, rear-side irradiance or ground-reflected radiation.
3. Required performance: Detector technology, ISO class, spectral response, temperature effect and response time.
4. Measurement range: Expected irradiance, configured analog span and logger scaling.
5. Electrical integration: Output, power supply, protocol, cable length and any external transmitter.
6. Calibration documents: Instrument serial number, calibration result, uncertainty, traceability and method.
7. Site conditions: Mounting, shading, contamination, dew, frost and maintenance access.
8. Service and ownership cost: Recalibration, accessories, spare parts and technical support.

The RK200-03 specifications list ISO 9847 as the calibration basis and a two-year recalibration interval. Request the instrument-specific certificate, including the method and edition used. ISO 9847:2023 describes calibration by comparison with a reference pyranometer.

For the RK200-04, establish the calibration documentation and service interval with RIKA for the intended application.